An electric power steering apparatus for applying a steering assist force to a steering mechanism of an automobile by a rotating force of a motor transmits a driving force of the motor by a transmitting mechanism such as a gear and a belt through a reduction gear to apply the steering assist force to a steering shaft or a rack shaft. A simple structure of such an electric power steering apparatus is shown in FIG. 12 and will be described with reference to the drawing.
A shaft 102 of a steering wheel 101 is coupled to tie rods 106 of steered wheels through a reduction gear 103, universal joints 104a and 104b, and a pinion rack mechanism 105. The shaft 102 is provided with a torque sensor 107 for detecting steering torque of the steering wheel 101 and a motor 108 for assisting a steering force of the steering wheel 101 is coupled to the shaft 102 through the reduction gear 103. The motor of the electric power steering apparatus is controlled by a control unit 109 by inputting a torque value T detected by the torque sensor 107, a vehicle speed V detected by a vehicle speed sensor (not shown), a rotation angle of the motor detected by a position detection sensor 110 for detecting a rotation position of the motor, and further a steering angle θs detected by a steering angle sensor 112 attached to the reduction gear 103, or the like to the control unit 109. The control unit 109 is mainly formed of a CPU and performs a motor control by using a program in the CPU.
The detected steering angle is used for an attitude control of the vehicle or used for controlling the electric power steering apparatus. Therefore, if the steering angle sensor comes into an abnormal condition, it is unfavorable to use the erroneous steering angle detected by the steering angle sensor for control and therefore, it is necessary to detect the abnormality of the steering angle sensor without delay. It is conceivable to form the steering angle sensor as a double system, but it increases the cost and thus other various abnormality detecting means have been devised.
There is a means for detecting an abnormality of a steering angle sensor as disclosed in Japanese Patent Application Laid-open (JP-A) No. 2002-104211, wherein a steering angle estimated from terminal voltage of a motor and motor current and a steering angle detected by the steering angle sensor 112 are compared with each other to detect the abnormality of the steering angle sensor.
There is one disclosed in JP-A No. 2003-252228, wherein a steering angle detected by a steering angle sensor 112 and a steering angle estimated from a motor rotation angle detected by a position detection sensor 110 for detecting the rotation position of the motor are compared to each other to detect an abnormality of the steering angle sensor.